Safeguarded electric firing initiating devices



Dec. 31, 1957 G. A. BURKLUND ,8

SAFEGUARDED ELECTRIC FIRING INITIATING DEVICES Filed Nov. 17, 1955 V 2 Sheets-Sheet 1 I N VE N TOR zf/v/v A aufikmA/o,

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G. A. BURKLUND SAFEGUARDED ELECTRIC FIRING INITIATING DEVICES Dec. 31, 1957 Filed Nov. 17, 1955 ATTORNEY United States Patent SAFEGUARDED ELECTRIC FIRING ]NITIATIN G DEVICES Glenn A. Burklund, Fairfax, Va.

Application November 17, 1955, Serial No. 547,486

11 Claims. (Cl. 102-28) This invention relates to improvements in electric firing initiating devices such as squibs, blasting caps and the like and more particularly to such devices containing means for safeguarding the devices against accidental and premature firing by the action of high frequency electrical energy.

Electrical firing initiating devices of the character to which this invention pertains, comprise, in general, a shell or casing closed at one end and open at the other in which is disposed a resistance bridge wire or other electrical heater element which is embedded in or located adjacent to an explosive igniting charge. Electrical conductors are connected to each end of the bridge wire or heater by which the heater is electrically energized by an external firing circuit to ignite the charge. A charge of heat sensitive explosive material such as mercury fulminate, lead azide or diazo dinitrophenol is employed in the firing device casing in heat initiating relation to the heater element which sensitive explosive material is first activated and which in turn ignites the base or secondary charge or an ignition charge if such amaterial is employed intermediate the sensitive explosive and the base charge. A water-proof composition in the nature of a packing is often positioned adjacent to the heat sensitive charge and a shell closure formed generally of heat flowable material is employed to seal the device and to embed the electrical conductor leads which extend therethrough to the bridge wire. It is customary for the conductor leads connected to the bridge wire or heater element to extend externally of the casing a distance of several inches up to a foot or more since these lead lengths serve to facilitate the connection of the device in a firing circuit.

Electrical firing initiating devices such as are described above are useful in igniting various explosive compositions used in many dilterent applications such as solid propellant rocket motors, bombs, seismic charges, land and sea mines, under water demolition explosives, jato devices, and other similar types of ordnance material and devices, as well as general earth blasting. With an electrical squib embedded in the solid propellant of a rocket or the explosive of a bomb, mine or the like, energization of the heater element by a firing circuit results in the heating of the bridge wire that serves in turn to cause the initiation of the heat sensitive charge which results in the initiating of the ignition of the base or secondary charge or an ignition charge, if one is used, which subsequently initiates the ignition of the base charge of the squib. The initiation of the base charge causes the squib casing to rupture and to ignite the rocket propellant, etc. or primer charge therefor.

The transportation of packaged electric firing initiation devices such as squibs and blasting caps and also the handling and use of ordnance devices such as rockets, bombs mines and other explosives in which such electric firing devices are assembled, has'become extremely hazardous due to the premature firing of the initiator which may accidentally occur by initiation of the sensitive explosive. When a squib or when an ordnance device with which a squib is utilized is positioned in or is passed through an electromagnetic field, this hazardous condition is multiplied many fold. Such a condition occurs when the squibsorordnance devices embodying the squibs are positioned inconcentrated pulsed beams of electromagnetic energy radiated by radar transmitters or missile guiding transmitters emitting modulated or unmodulated high frequency carrier waves or pulses of energy. .Thus, such a beam of electromagnetic energy impinging upon a military plane or vessel carrying rocket motors or other ordnance devices equipped with electrical squibscould readily induce sufiicient high frequency voltage in the conductor leads of the squib or in the planes or ships rocket firing circuit which is connected to the squib to cause the bridge wire to be energized to ignite the sensitive explosive charge and in turn cause the firingof the squibs secondary charge which would result in the premature firingof rocket motors, jato devices, bombs or mines carriedby the plane or other military vehicle. If the squibs were not assembled in the ordnance device, the explosion of a squib by itself or with others in packaged form would be injurious to personnel in thevicinity of the explosion. Additionally, a moving beam or a burst of high frequency electromagnetic energy impingingupon a guided missile in firing position on a land or naval vessel supported launcher could induce voltagcs'inthe umbilical cable which would serve prematurelyto initiate the squib or squibs and result in the firingof the guided missile.

Many suggestions have been made for protecting electrical firing initiators against accidental and premature firing. One of the simplest of these is the concept of short circuiting-the conductors used to energize the bridge wire, thus making it impossible accidentally to energize the heater elementby induced currents or by a battery or the like being placed across the conductors. Another method which has been suggested for preventing premature firing is the use ofalmaterial or a device positioned in series with the-bridgcwire circuit which otters a high resistance to the flow of unwanted currents but permits the firing current to How in the bridge wire circuit. Still other suggestions have'beenmade concerning the establishment of a conductive circuit through coherer action which permits the by-passing of unwanted electrical currents and charges therethrough and thus eliminates the flow of such charges through the bridge wire and thereby maintains the firing initiating device against accidental firing.

All of the solutions previously suggested for eliminating the premature firing of such firing initiating devices have been accompanied by numerous disadvantages which have'in general, made their use undesirable with the possible exceptionof the use of physical short circuiting devices. Itis apparent that at a, time prior to the firing of an ordnance device, the short circuit must be physically removed fromthe initiating device and when this'is done thedevice thus becomes subject to the danger of premature firing as above described. Further, even though the metallic ends of the squib leads are electrically connected together for safeguarding during transportation, premature firing may take place due to the inducing of a voltage in the closed loop of the squip conductors by a stronglfiuctuating magnetic field. One of the very material disadvantages of the solutions heretofore suggested: is. the inherent reduction of the sensitivity during, the period of protection. Thus the concept of maintainingrthe designed sensitivity level of firing and the concept of safeguarding against premature and accidentalinitiation were heretofore considered to be muelectrical firing initiating devices which are safeguarded against accidental or premature firing without reducing the degree of sensitivity of the devices to initiating currents such as are supplied by firing circuits. This is accomplished by providing the initiating device with 'a by-pass or shunt circuit selectively to conduct the flow of high frequency currents therethrough and away from the bridge wire while stopping the flow of direct firing currents there through and diverting such currents to flow through the bridge Wire. By the use of the described circuit which bypasses or shunts the firing element, the initiating devices are safeguarded against harmful flow of induced high frequency currents and at the same time retain their senlsllivily indir ct cu rei fi flo n fir n fi i l ,1? l

greater particularity, the improved electrical firing initiating devices of this invention are protected by electrically inserting across the bridge wire leads thereof a capacitor which provides a low impedance path for high frequency currents and provides an infinitely high resistance, if not a complete barrier, to the flow of currents normally employed in electrical squib and blasting cap firing circuits.

Another object of the invention is to provide an improved electrical firing initiating device wherein a safeguarding capacitor connected across the conductors of the bridge wire is supported by the device and positioned entirely within the shell or casing of the device without the need to change the physical dimensions of the conventionally used casings.

Another object of the invention is to provide an improved electrical firing initiating device provided with a shunt connected capacitor that is positioned entirely within the casing of the device, which capacitor may be formed of (l) spiral wound flexible electrodes spaced apart by a flexible dielectric material, (2) sandwiched plates or wafers of conducting and dielectric materials which may have either flexible or rigid characteristics, and (3) tubular shaped electrodes separated by similarly shaped dielectric material.

Another object of the invention is to provide improved electrical firing initiating devices provided with a capacitor electrically shunted across the bridge wire, which capacitor is formed of a tube of insulating material with the electrode elements formed of thin metallic layers positioned on opposite sides of the tube.

Another object of the invention is to provide improved electrical firing initiating devices wherein a capacitor connected across the bridge wire of the device is formed of a hollow cylinder with metallic electrodes located on both sides theerof, which composite unit serves as a housing or enclosure for the electrical firing device and explosive charges.

A further object of this invention is to provide an improved electrical firing initiating device having a capacitor for conducting high frequency currents electrically connected in parallel relation to the bridge wire, which capacitor is formed of a material having a high dielectric constant, such for example, as barium titanate, lead zirconate-titanate compositions, barium-strontium titanate, titanium dioxide, and the like, in order that high capacity can be achieved with relative small sized units.

A further object of this invention is to provide an improved electrical firing initiating device provided with a capacitor electrically connected across its bridge wire, which capacitor is positioned above the explosive charge and serves as a closure plug of the shell or casing.

A still further object of the invention is to provide an improved electrical firing initiating device provided with an open-ended metallic casing and a capacitor electrically connected across the bridge wire, wherein one or more of the capacitor electrodes serves as a stray field shielding means for said open end of the casing.

A still further object ofthe invention is to provide an improved balanced electrical firing initiating device provided with a metallic casing wherein each of the bridge 4 wire conductors are grounded to the metallic casing through individual capacitors.

A still further object of the invention is to provide an improved electrical firing initiating device provided with a tubular shaped capacitor which is electrically connected in parallel relation to the heater element of the device and which serves as casing or shell for the device.

A still further object of the invention is to provide an improved electrical firing device having a tubular shaped shell or casing formed of a dielectric material on each of the sides of which is positioned a capacitor electrode and having an electrode connection extending between the opposed electrodes and opposite bridge wire conductors.

The invention also resides in certain novel structure characteristics and features which facilitate the carrying out of the foregoing objects and which contribute both to the safeguarding of the electrical firing device against accidental and premature firing and to the packaging of the safeguarded devices in conventional sized shells or casings.

Other objects and advantages of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings, in which:

Fig. 1 is a Wiring diagram showing the parallel circuit arrangement of the capacitor unit with respect to the bridge wire heating element used in the safeguarded firing initiating device;

Fig. 2 is a cross-sectional view showing an improved safeguarded electrical firing initiating device such as a squib or a blasting cap having a multi-plate capacitor positioned within the casing and electrically connected across the conductor leads of the heating element;

Fig. 3 is a cross-sectional view showing the upper portion only of a capacitor equipped device of the general type shown in Fig. 2 wherein the shunt connected capacitor is spirally wound and is positioned within the casing with its axis substantially parallel to the axis of the casing;

Fig. 4 is an elevational view of the spiral wound capacitor and surrounding insulating sleeve shown in Fig. 3;

Fig. 5 is an enlarged cross-sectional fragmentary view of the flexible electrodes and dielectric layers of the spirally wound capacitor shown in Figs. 3 and 4;

Fig. 6 is a schematic wiring diagram showing a heater element of a safeguarded electric firing initiating device provided with a shunt connected capacitor as in Fig. l and in addition thereto a balanced capacitor network which serves to ground each of the heater element conductor leads to the casing of the device through an individual capacitor;

Fig. 7 is a cross-sectional view showing the upper portion only of a safeguarded firing initiating device of the general type shown in Fig. 2 wherein a plurality of capacitors, shown in elevation, are electrically connected in the circuit arrangement of Fig. 6;

Fig. 8 is an enlarged cross-sectional fragmentary view of the individual capacitor units, heater element and heat sensitive explosive charge shown in Fig. 7;

Fig. 9 is a cross-sectional broken view of an improved electrical firing initiating device wherein a safeguarding tubular shaped capacitor, connected across the leads of a bridge wire, is formed of an insulating cylinder of dielectric material and has metallic condenser plates positioned on each side thereof, which cylinder and plates fit within the casing of the device and are spaced from the walls thereof by a relatively thin insulating sleeve;

Fig. 10 is a cross-sectional view showing the upper portion only of safeguarded firing initiating device of the general type shown in Fig. 9 wherein the metallic shell or casing of the device serves as the outer plate or electrode of the tubular shaped capacitor, and

Fig. 11 is a cross-sectional view showing the upper portion only of a firing initiating device safeguarded by a tubular type capacitor wherein the casing of the device is formed of insulating material which serves as the didrawings and will herein describe in detail the preferred embodiments. It will be understood, however, that I do not intend to limit the invention by such disclosures for I aim to cover all modifications and alternative constructions falling within the spirit and scope of the invention as defined in the appended claims.

Referring to Fig. 1 of the drawings, reference numerals L and L indicate conductor leads connected to the heater element 12 of a firing initiating device such as an electrical squib or blasting cap. A safeguarding capacitor 14 is connected across spaced conductors L and L and in shunting or parallel relation to the heater element 12. When the spaced conductors L and L are connected to a source of direct current voltage for the purpose of heating the bridge wire to activate the heat sensitive explosive material of the device, no current fiow occurs through the parallel positioned capacitor since it provides infinitely high resistance to the flow of such current and, accordingly, the sensitivity of the firing device is not depressed. However, if L and L are energized by a source of high frequency energy either by connection to such a source or by having such energy induced therein, the high frequency energy will be by-passed through the shunt connected capacitor since the capacitor circuit offers low impedance to the flow of the high frequency current and thus the heater element will not be energized to activate the sensitive explosive material. The ranges of high frequency energy by-passed will be dependent upon the capacitance of the capacitors utilized and will comprise wavelengths of the order of 30,000 meters to a small portion of a centimeter corresponding to a he quency range of about kc. to about 1,000,000 mc.

Heater element 12 is provided with a bridge formed of resistance wire or foil having a positive resistance characteristic to cause its temperature to increase upon the flow of electrical current therethrough.

The capacitor 14 of Fig. 1 may be formed in various configurations to enable it to be supported by the casing of an electrical firing initiating device. The capacitor is formed with a configuration to enable it to be positioned within the generally cylindrical shaped casing of a squib or blasting cap although I have disclosed an en1- bodiment wherein the capacitor itself serves to form the casing of the device and to house the explosive charges and heating element. Thus for the purpose of this invention capacitor 14 is preferably formed (1) by a plurality of flat electrode plates spaced by dielectric material, (2) by spirally wound flexible electrodes and a dielectric layer therebetween, and (3) by tubular shaped electrodes having intermediately positioned dielectric material. When the capacitor is formed with a tubular shape, the cylindrical metal or insulating case of the electrical firing initiator may conveniently form one element of the capacitor as will be described in detail.

A capacitor designated generally by reference numeral 24 in Fig. 2 is formed of a plurality of stacked flat metallic electrodes 20 and 22 separated by layers of dielectric material 21 having a configuration to permit the capacitor to be. positioned within the shell of the electrical firing device. Plate electrodes 20 are connected to conductor L and plate electrodes 22 are connected to conductor L Reference numeral 26 indicates the cylindrical shell or casing of an electrical squib or blasting cap which shell is formed of a metal such as copper, aluminum, iron, or an alloy thereof. The shell is closed at the bottom end 28 and open at the opposite or top end 30. A base charge of explosive material 32 is contained within the lower portion of casing 26 and superimposed thereon is a firing assembly including a primary charge 34 of heat sensi- .6 tive explosive material which is capable of initiating the base charge 32. The heat sensitive explosive material is selected from such materials well-known in the art, viz. lead azide, boron and red lead mixture, mercury fulminate, diazo dinitrophenol, and the like. A heater element in the form of a bridge wire 12 is imbedded in or located in heat exchange relation to primary charge 34 and is connected between the ends of conductors L and L which extend into the interior of the casing 26 of the electrical firing initiating device. As is shown in Fig. 2, capacitor 24 is positioned Within casing 26 above the primary charge 34 and the opposite electrodes 20 and 22 of the sandwiched plate-type capacitor 24 are connected alternately to spaced conductors L and L with layers of dielectric material 21 interposed between the capacitor plates of opposite polarity. The microfarad capacity of condenser 24 may be increased or decreased by either increasing or decreasing the number of plates of opposite polarity and, as is clearly shown in Fig. 2, the opposite plates are connected, irrespective of the number thereof, to conductors L and L The individual plates or electrodes 20 and 22 of capacitor 24 may be formed of metallic foils or layers such, for example, as aluminum, gold, silver, which may be prepared by rolling operations or by electroplating. The dielectric material positioned intermediate the capacitor plates of opposite polarity is preferably a material having a high dielectric constant such, for example, as barium titanate, lead zirconate-titanate compositions, barium-strontium titanate, titanium dioxide, and the like, although other insulating materials such as mica, glass, and paraffin impregnated fibrous materials may be used. Preferably after assembly of the plates 20 and 22 and layers of dielectric material 21 of capacitor 24 and connecting the plates of opposed polarity to spaced conductors L and L the unit is imbedded in an insulating plug 36 of material such as an epoxy resin for the purpose of enclosing and supporting the components of the capacitor and for aiding in the handling of the unit. The plug 36 of resinous material in which capacitor 24 is imbedded is preferably superimposed above the primary mixture 34 and, as shown in Fig. 2, the plug of imbedding material is formed to fit snugly within the cylindrical casing of the electrical firing initiating device. Conductors L and L extend externally of the electrical firing initiating device and are provided With insulating covers 38 and 40. In order to prevent moisture and other foreign substance from gaining access to the interior of the electrical firing initiating device including the base and primary charges 32, 34 as Well as the capacitor components, a plug 42 of sealing material is positioned to close the open end 30 of the firing initiating device. The conductors L and L may terminate in electrical contact buttons positioned on the upper face of plug 42 which buttons are engaged by a suitable connector of a firing circuit. The use of such contact buttons eliminates the need for extended external L and L leads and thus reduces the possibility of currents being induced therein by radio frequency energy. An electrical squib or blasting cap provided with a high frequency current by-passing capacitor 24 of the character described is entirely self-contained and the safeguarding capacitor 24 and its electrical connections to conductors L and L are protected Within the metallic casing 26 of the device against injury.

In order that the operation of the capacitor protected electrical firing initiating device may be more fully un derstood, the following exemplary data is presented for explanation and without limitation with respect to the use of a ten-plate capacitor to safeguard an electrical squib against accidental premature firing by radiated high frequency electrical energy having a frequency range of between 60 and 65 megacycles. The data is based upon the use of insulation having a dielectric constant of 10,000, dielectric thickness of .015 inch, and a plate 7 or electrodediameter of substantiallyti inch. By substituting in the following equation wherein K=Dielectric constant S=individual plate area in centimeters squared C =capacitance in micro-microfarads t=plate separation in centimeters a capacitance per unit of .0041 microfarads is obtained or a total capacitance value of .041 microfarads is obtained for the build-up capacitor. This composite unit will thus provide a capacitive reactance of .061 ohms based upon a frequency in the mid-range of 60-65 megacycles and on the basis of an assumed resistance in the bridge wire 12 of 1 ohm, only approximately 6% of the total unwanted radio frequency current will flow through the bridge wire of heater 12 as compared with 100% flow if the safeguarding capacitor 24 is not employed. From the data presented it will be apparent that the use of the l-plate capacitor electrically connected across the heater element provides a 25 decibels attenuation in the flow of current through the bridge wire heating element resulting from the high frequency radiation. Reduction of current flow in the bridge wire in the manner above discussed prevents firing of the heat sensitive charge and thus eliminates accidental and inadvertent firing of the safeguarded squib or blasting cap by the radiated high frequency energy. it will be apparent that by following the teachings of this invention, a capacitor may be utilized which will safeguard the firing initiating device against accidental and inadvertent firing by high frequency wave energy.

Figs. 3 and 4 represent a modified form of the capacitor unit mounted within the casing 26 of the electric firing initiating device. In Figs. 3 and 4 capacitor 44 corresponds in function to capacitor 24 of the embodiment shown in Fig. 2, but an alternative form of capacitor is employed. Capacitor 44 is formed by spirally winding in a tight formation upon itself two flexible metallic electrodes 46, 48 and a oo-existing intermediate layer of flexible dielectric material 50 together with an insulating spacing layer 51. A cross-section of the spiral wound capacitor 44 is shown in Fig. 4 wherein spaced flexible metal electrodes 46 and 48 are spirally wound together with a layer of flexible dielectric material 50 which may comprise paraffin impregnated cellulose fibers in the form of paper or natural or synthetic resin impregnated thin sheet material. Conductors L and L after extending through the plug of sealing material 42 are connected at 52 and 54 respectively to electrodes 46 and 48 of the capacitor 44. Opposite sides of the heater bridge wire 12 of the embodiment shown in Fig. 3 are connectedrespectively to electrodes 46 and 48 by connecting wires 56 and 58 respectively. By this structure and connections, the spirally wound capacitor 44 is electrically connected in parallel relation with bridge wire 12 and, as is shown in Fig. 3, the wound condenser is preferably formed with a diameter to fit within the cylindrical casing 26 of the firing device and is spaced from the wall of the casing by an insulating tube section 60. In other respects the firing squib and blasting cap structure of Figs. 3 and 4 are similar to the structure of the device of Fig. 2 which has heretofore been described in detail.

A multiple capacitor network is disclosed in Fig. 6 in addition to protective-capacitor 14 wherein provision is made for dissipating high frequency potentials induced in either one or both of conductors L and L through separate capacitor containing paths to the casing of the device. Inasmuch as the metallic casing of electrical firing initiating devices ordinarily exists at ground potential, it sometimes happens that arcing occurs betweenone ofihe bridge wire. conductors and the case uponthe occurrence of in ficient to activate the sensitive primary mixture thus causing the inadvertent, premature and accidental firingof the initiator. By use of the balanced system disclosed in Fig. 6 the high frequency currents induced in spaced conductors L and L are led to the casing ground'through individualcapacitors 61 and 62. Thus the firing device is protected irrespective of which lead-in conductor has induced therein high frequency potentials. If such potentials are induced in conductor L the resultant current flow will be by-passed to groundthrough capacitor 61 and in a similar manner high frequency current flowing in conductor L will be'by-passed to ground through capacitor 62. Capacitor 14 is connected across conductors L L in parallel relation to bridge wire 12 and, accordingly, it serves a function similar to capacitors 24 and 44 which have been previously described.

A physical arrangement of the protective system of Fig. 6 is shown in Fig. 7 wherein a plurality of wafer type in Fig. 2. However, in the multiple capacitor embodiment of Fig. 7, it has been found convenient to form aligned apertures in the capacitor units 71, 72 and 74 and pass conductors L and L therethroug h. The individual electrodes of the respective capacitors are connected to conductors L and L as shown in Fig. 6 by the use of welding, brazing or soldering. It will be understood that additional plates may be employed in connection with the grounding capacitors 71 and 72 if desired and, in addition, capacitor 74 which shunts the bridging circuit may have an additional number of plates as is shown in Fig. 2.

Fig. 8 shows in greater detail the arrangement of capacitors and the connection thereto of conductors L and L employed inthe electrical firing initiating device .of Fig. 7. Plate 76 of capacitor 71 and plate 78 of capacitor 72 are connected to casing 26 at 73. Conductor L is connected at 70 with plate 80 of capacitor 71 and at 81 with plate 82 of capacitor 74. Conductor L is connected at 77 with plate 79 of capacitor 72 and at 83 with plate 84 of capacitor 74. The capacitors and connections thereto are imbedded ina plug 36 of plastic material in a manner similar to capacitors 24 and 44 of Figs. 2 and 3..

An elongated tubular shaped capacitor is utilized in the electric firing initiating device disclosed in Fig. 9. The axis of the tubular shaped capacitor represented generally by reference numeral 80 coincides with the lengthwise axis of casing 26 and the capacitor has an outside diameter tofit snugly within the insulating sleeve 84 which is positioned between capacitor 80 and the interior wall surface of casing 26. Capacitor 80 is formed of a tubular shaped dielectric element 86 to the inner and outer surfaces of which are applied metallic layers 88 and 90. The metallic layers 88 and 90 may comprise foil or thin coatings deposited in any suitable manner, such for example as is disclosed in Patent 2,525,668 to Robert B. Gray dated October 10, 1950. A disc 92 of insulating material is positioned at the bottom of casing 26 shown in Fig. 9 which eliminates the possibility of short circuiting metallic layers 88 and 90 by contact with bottom 28 of the casing. The tubular shaped capacitor disclosed in Figure9 is connected in parallel arrangement with a heater element 12 by the use of tie wire conductor 94 which is connected between metalliclayer 90 and conductor L 88 and conductor L By the structure disclosedinFig. 9,

is formed of insulating material which serves as the dielectric material of a capacitor embodying inner and outer metallic plates which capacitor is connected in parallel relation to the heater element of the initiating device.

While the invention is susceptible of various modifications and alternative constructions, I have shown in the drawings and will herein describe in detail the preferred embodiments. It will be understood, however, that I do not intend to limit the invention by such disclosures for I aim to cover all modifications and alternative constructions falling within the spirit and scope of the invention as defined in the appended claims.

Referring to Fig. l of the drawings, reference numerals L and L indicate conductor leads connected to the heater element 12 of a firing initiating device such as an electrical squib or blasting cap. A safeguarding capacitor 14 is connected across spaced conductors L and L and in shunting or parallel relation to the heater element 12. When the spaced conductors L and L are connected to a source of direct current voltage for the purpose of heating the bridge wire to activate the heat sensitive explosive material of the device, no current flow occurs through the parallel positioned capacitor since it provides infinitely high resistance to the flow of such current and, accordingly, the sensitivity of the firing device is not depressed. However, if L and L are energized by a source of high frequency energy either by connection to such a source or by having such energy induced therein, the high frequency energy will be by-passed through the shunt connected capacitor since the capacitor circuit offers low impedance to the flow of the high frequency current and thus the heater element will not be energized to activate the sensitive explosive material. The ranges of high frequency energy by-passed will be dependent upon the capacitance of the capacitors utilized and will comprise wavelengths of the order of 30,000 meters to a small portion of a centimeter corresponding to a frequency range of about kc. to about 1,000,000 mc.

Heater element 12 is provided with a bridge formed of resistance wire or foil having a positive resistance characteristic to cause its temperature to increase upon the flow of electrical current therethrough.

The capacitor 14 of Fig. 1 may be formed in various configurations to enable it to be supported by the casing of an electrical firing initiating device. The capacitor is formed with a configuration to enable it to be positioned within the generally cylindrical shaped casing of a squib or blasting cap although I have disclosed an embodiment wherein the capacitor itself serves to form the casing of the device and to house the explosive charges and heating element. Thus for the purpose of this invention capacitor 14 is preferably formed 1) by a plurality of fiat electrode plates spaced by dielectric material, (2) by spirally wound flexible electrodes and a dielectric layer therebetween, and (3) by tubular shaped electrodes having intermediately positioned dielectric material. When the capacitor is formed with a tubular shape, the cylindrical metal or insulating case of the electrical firing initiator may conveniently form one element of the capacitor as will be described in detail.

A capacitor designated generally by reference numeral 24 in Fig. 2 is formed of a plurality of stacked flat metallic. electrodes 20 and 22 separated by layers of dielectric material 21 having a configuration to permit the capacitor to be positioned within the shell of the electrical firing device. Plate electrodes 20 are connected to conductor L and plate electrodes 22 are connected to conductor L Reference numeral 26 indicates the cylindrical shell or casing of an electrical squib or blasting cap which shell is formed of a metal such as copper, aluminum, iron, or an alloy thereof. The shell is closed at the bottom end 28 and open at the opposite or top end 30. A base charge of explosive material 32 is contained within the lower portion of casing 26 and superimposed thereon is a firing assembly including a primary charge 34 of heat sensitive explosive material which is capable of initiating the base charge 32. The heat sensitive explosive material is selected from such materials well-known in the art, viz. lead azide, boron and red lead mixture, mercury fulminate, diazo dinitrophenol, and the like. A heater element in the form of a bridge wire 12 is imbedded in or located in heat exchange relation to primary charge 34 and is connected between the ends of conductors L and L which extend into the interior of the casing 26 of the electrical firing initiating device. As is shown in Fig. 2, capacitor 24 is positioned within casing 26 above the primary charge 34 and the opposite electrodes 20 and 22 of the sandwiched plate-type capacitor 24 are connected alternately to spaced conductors L and L with layers of dielectric material 21 interposed between the capacitor plates of opposite polarity. The microfarad capacity of condenser 24 may be increased or decreased by either increasing or decreasing the number of plates of opposite polarity and, as is clearly shown in Fig. 2, the opposite plates are connected, irrespective of the number thereof, to conductors L and L The individual plates or electrodes 20 and 22 of capacitor 24 may be formed of metallic foils or layers such, for example, as aluminum, gold, silver, which may be prepared by rolling operations or by electroplating. The dielectric material positioned intermediate the capacitor plates of opposite polarity is preferably a material having a high dielectric constant such, for example, as barium titanate, lead zirconate-titanate compositions, barium-strontium titanate, titanium dioxide, and the like, although other insulating materials such as mica, glass, and parafiin impregnatecl fibrous materials may be used. Preferably after assembly of the plates 20 and 22 and layers of dielectric material 21 of capacitor 24 and connecting the plates of opposed polarity to spaced conductors L and L the unit is imbedded in an insulating plug 36 of material such as an epoxy resin for the purpose of enclosing and supporting the components of the capacitor and for aiding in the handling of the unit. The plug 36 of resinous material in which capacitor 24 is imbedded is preferably superimposed above the primary mixture 34 and, as shown in Fig. 2, the plug of imbedding material is formed to fit snugly within the cylindrical casing of the electrical firing initiating device. Conductors L and L extend externally of the electrical firing initiating device and are provided with insulating covers 38 and 40. In order to prevent moisture and other foreign substance from gaining access to the interior of the electrical firing initiating device including the base and primary charges 32, 34 as well as the capacitor components, a plug 42 of sealing material is positioned to close the open end 30 of the firing initiating device. The conductors L and L may terminate in electrical contact buttons positioned on the upper face of plug 42 which buttons are engaged by a suitable connector of a firing circuit. The use of such contact buttons eliminates the need for extended external L and L leads and thus reduces the possibility of currents being induced therein by radio frequency energy. An electrical squib or blasting cap provided with a high frequency current by-passing capacitor 24 of the character described.

is entirely self-contained and the safeguarding capacitor 24 and its electrical connections to conductors L and L are protected within the metallic casing 26 of the device against injury.

In order that the operation of the capacitor protected electrical firing initiating device may be more fully understood, the following exemplary data is presented for explanation and without limitation with respect to the use of a ten-plate capacitor to safeguard an electrical squib against accidental premature firing by radiated high frequency electrical energy having a frequency range of between 60 and 65 megacycles. The data is based upon the use of' insulation having a dielectric constant of 10,000, dielectric thickness of .015 inch, and a plate 01? cli$trod "-dia i'neter' of substantially $33; incli l f sub stitutingfin'th'e' following equation wherein:

K- =Dielectric constant S-' individual plate area in centimeters squared C=capacitance in micro-microfarads t=plate separation in centimeters a capacitance per unit of .0041 microfarads is obtained or--.a totalcapacitance .value of. .041 microfarads is. obtainedfor the build-up capacitor. This composite unitv will thus provide a capacitive 'reactance .of- .061' ohms. based upon a frequency in the mid-range of 60-65 megacyclesand on the basis of an assumed resistance in the bridge wire 12 ofl ohm,.only approximately6% of .the total unwanted radio frequency current will flow through the bridge wire of heater 12 as compared with 100% flowi-if the safeguarding capacitor 24- is not employed. From the data presented it will be apparent that ,the use of the l-platecapacitor electrically connected across. the heater element provides a 25 decibels attenuation in the. flow of current through the bridge wire heating element resulting from the high frequency radiation. Reduction of current flow in the bridge wirein the manner above discussed prevents firing of the heatsensitive charge and thus eliminates accidental and inadvertent firing of the safeguarded squib or blasting cap by the radiated high frequency energy. It will be apparent that by fol-. lowing the teachings of this invention, a capacitorrnay be ntilized which will safeguard the firing initiating device against accidental and inadvertent firing by high frequency wave energy.

Figs. 3 and 4 represent a modified form of the capacitor unit mounted within the casing 26 of the electric firing initiating device. In Figs. 3 and 4 capacitor 44 corresponds in function to capacitor 24 of the embodiment shown in Fig. 2, but an alternative form ofcapacitor is employed. Capacitor 44 is formed by spirally winding .in a tight formation upon itself two flexible metallic electrodes 46, 48 and a co-existing intermediate layer of. flexible dielectric material 50 together with an insulating spacing layer 51. A cross-section of the spiral wound capacitor 44. is shown inFig. 4 wherein spaced flexible metal electrodes 46 and 48 are spirally wound together with .a layer of flexible dielectric material 50 which may comprise paraffin impregnated cellulosefibe'rsin the form of paper or natural or synthetic resinimpr e'gnated tliin sheet material. Conductors L and L after extending through the plug of sealing material 42 are; connectedat 52 and54 respectively to electrodes 46 and 48 0f the capacitor 44/ Opposite sides of the heater bridge wire 12 of the embodiment shown in Fig. 3 are connected 're spectively to electrodes 46 and 48 by connecting wires 56 and 58 respectively By this structure and connections, the spirally wound capacitor 44 is electrically connected in parallel relation with bridge wire 12 and, as is shown in Fig. 3, the wound condenser is preferably formed with a diameter to fit within the cylindrical casing 26- of the firing device and is spaced from the wall of the-casing by an insulating tube section 60.- In otherrespects the firing squib and blasting cap structure of Figs. 3 and 4 are similar to the structure of the device of Fig; Z-which has heretofore been described indetail.

A multiple capacitor network is-disclosed in Fig. 6 in addition to protective capacitor 14 whereinprovi'sion is made for dissipatinghigh frequency potentials induced in either one or both ofconductors L and L through s' e'parate capacitor containing paths to the casing'of the device. Inasmuch as the metallic fcasing of 'electrical'firing initiat ing devices ordinarily-exists-at ground potential, -it "'so'r'rietinies liapp'ens that-arc'ihg occursberw'eeneaeer the bridge wii e -eondu'c'tcrs ann theca'se upen rlie occurrence ofiin "an additional number of plates as is shown in Fig. 2.

' bedded in {a plug 36 of plastic material in a manner. similar 8 dueans f"queney wn1rages.-; The heat generatedgdurg in'g 'e else the-initiator By use of the balanced' systen diselosed in Fig; 6 the high frequency currents induced spaced' conductors L and'L are led to the casing ground through tentials are induced in conductor L the resultant'current; flow will be by-pass'ed to ground through capacitor 61 andin a simil'armanner high frequency current flowing in condu'ctor' L will be by-passed to groundthrough capacitor- 62 Capacitor 14is connected acrossconductors 111, 1.5 in parallel relation --to bridge wire 12 and, accordingly; it

serves a function similar to capacitors 24 and '44 which have, been previously described.

A- physical arrangement of the protective system of Fig. 6 is shown in Fig. 7 wherein a plurality of wafer type capacitors are employed. The capacitor units 71 and -72 I haveone electrode thereof grounded to casing 26 and the;

other electrodes respectively connected to conductors L and L and the capacitor inserted across spaced conductors :L and L in parallel arrangement to bridge wire 12 is designatediby reference numeral 74. Aside from the number of and particular connections of the respective capacitors 71,7 72 and 74," the arrangement of parts is similar to the electrical firing initiating device disclosed iri Fig'. 2. However, in the multiple capacitor embodi ment of Fig. 7, it has been found convenient to form aligned apertures in the capacitor units 71, 72 and 74 and pass conductors L and L therethrough. The individual electrodes of the respective capacitors are connected to conductors L andL as shown in Fig. 6 by the use of welding, brazing or soldering. It will be .understood that additional plates may be employed in connection with the grounding capacitors 71 and 72 if desired and, in addition, capacitor 74 which shunts the bridging circuit may have Fig. 8 shows in greater detail the arrangement of capacitors and the connection thereto of conductors L and L employed in the electrical firing initiating device of Fig. 7. Plate 76 of capacitor 71 and plate 78 of capacitor 72 are connected to casing 26 at 73. Conductor L is connected at with plate 30 of capacitor 71 and at 81 with plate. 82' of capacitor 74. Conductor L is connectedat 77 with plate 79 r capacitor 7; and at 83 with plate'84 of capaci tor 74. The capacitors and connections thereto are imto capacitors 24 and 44 of- Figs. 2 and3.

An elongated tubular shapedcapacitor is utilizedin the electric firing initiating device disclosed in Fig. 9. The .axis of the tubular shaped capacitor represented generally 1 by'refei ence'numeral coincides with the lengthwise of casing 26 and the'capacitor has-an outside diameter to fit snugly within the insulating sleeve 84 which is positioned between capacitor 80 and the interior wall surface .of casing 26. Capacitor 80 is for-medof a tubular shaped dielectric element 86 to the inner and outer surfaces of whichareapplied metallic layers 88 and 90. The metallic layers 88: and 90 may comprise foil or thin coatings deposited in any suitable; manner, suchfor example as is disclosed in Patent.2, 525,668 to Robert B. Gray dated October 10,1950. A disc 92 of insulating material .is. positioned at the bottom of casing 26 shown in Fig. 9 3

which eliminates the possibility of short cireuiting metallic layers 88 and; by contact with bottom 28 of the .casing. The tubulanshaped capacitor disclosed in Figure; 9

is connected. in parallel) arrangementwith a heater element-12 by the use'of tiewire conductor 94 which is connected' 'between metallic layer 90 and conductor L andtie wire 96'whi'chis connected between {metallic layer- 88; and conddctoril..,.. By thersfrnctnre 'disclosed in Fi A A at dischargehasbeenknown-rehearse: ficient-ft'o activate the sensitive primary mixture'thus causing'the inadvertent; premature and accidental firing bf the tubularshaped; capacitor'iscompletely insulated from the CflSlllg the-firinginitiatingjdeviceand the space within the interior of the capacitor is utilized. to contain thefiring assembly primary explosive charge andthe secondary charge of the device.-

An electrical firing initiating device utilizing an elongated tubular shaped, capacitor is-shown in Fig. 10. In this embodiment casing26' of the firing initiating device is utilized as the outside plate or electrode of the tubular capacitor. A tubular shaped, initiating member 102 which forms the dielectric of thecapacitor is positioned interiorly of the casing in. contacting relation with the wall thereof. A metallic layer 104 is positioned interiorlyof the tube of.dielectricmateriali-to form the opposite plate or electrode of the capacitor. Tic wire 106 is shown connected betweencasing 26 of the electric firing initiating device and conductor L and tie wire 1% serves to electrically connect the electrode 104with conductor L An alternative embodiment is disclosed in Fig. 11 wherein the casing 110 is formed of an insulating material which serves as the dielectric for the capacitor which is connected in parallel relationshipto heater element 12. A metallic layer 112 is positioned on the outside surface of the insulating casingand serves as one plate or electrode of the capacitor. Metallic layer 114 is positioned on the inside: surface. of insulating casing 110 and serves as the other plate or electrode of the capacitor. The outside electrode 112 is connected to conductor L by tie wire 116 which passes through an aperture 118 in the insulating casing 110 as shown in Fig. 11. The aperture is preferably filled around tie wire 116 by the use of any suitable congealable sealing material. The interiorly positioned electrode 114 is connccted to conductor L by electrical tie wire 120, and by the use of tie wires 116 and 120 the capacitor which is formed integrally with the insulating casing of the firing initiating device is connected across conductors L and L and in parallel relationship to heater element 12. An element (not shown) such as is indicated at 92 in Fig. 9 serves to close the bottom end of casing 110. This element is formed of insulating material and is maintained in position by the use of adhesive material it formed separate from the casing, or the end element 28 may be formed integral with the insulating casing 110 at the time it is cast.

The dielectric materials utilized in the capacitors of Figs. 9 to 11, inclusive, may be those previously mentioned in connection with capacitor 24 of Fig. 2. When insulating ceramic materials are utilized which generate voltages upon being mechanically vibrated or placed in stress, such for example as titanate materials, previously mentioned, it will be necessary to eliminate the efiect of such voltage generation by arranging the crystals in opposed relationship to efiect voltage cancellation and thus eliminate the possibility of accidental and premature firing of the initiating device by current flow resulting from voltages generated in the dielectrical material by piezoelectric elfect. Such techniques are known in the piezoelectric crystal art and form no [part of the present invention.

It will be apparent that the tubular capacitor units disclosed in Figs. 9 to 11, inclusive, may be formed of any desired length to effect the desired capacitive reactance in the circuits of Figs. 1 and 6. Additionally; a plurality of tubular capacitors may be positioned in end to end relation and plate-type capacitors may be employed in conjunction with tubular formed capacitorswithout departing from the scope of this invention.

I claim:

1. An electric firing initiating device safeguarded against premature firing resulting from currents induced by high frequency energy comprising, in combination, a casing, an ignition assembly disposed within said casing comprising a pair of spaced conductors, a heater element connected in bridging relation across said conductors and a heat sensitive ignition composition disposed in heat ex- 10 change relation to said heater element, a CtlPitCllOl' Slip" ported by said casing, and electticalconnections located between each of said spaced conductors and the respective electrodes of said capacitor to electrically connect the capacitor in parallel relation to said heater element, whereby alternating currents are by-passed through said capacitor to safeguard the device without affecting its sensitivity to direct firing currents.

2. An electric firing initiating device as defined in claim 1 wherein said capacitor is positioned within the casing of said initiating firing device.

3. An electric firing initiating device as defined in claim 1 wherein an insulating material having a high dielectric constant is positioned between said opposed electrodes, said material being selected from the group of dielectric materials consisting of barium titanate, lead zirconate-titanate, barium-strontium titanate, titanium dioxide, mica, glass and parafiin impregnated fibrous material.

4. An electric firing initiating device as defined in claim 1 wherein the capacitor is formed of a plurality of condenser units each of which is formed by spaced, platetype, substantially flat electrodes separated by a layer of dielectric material.

5. An electric firing initiating device as defined in claim 1 wherein the capacitor is formed of a plurality of tightly wound convolutions of elongated thin, flexible layers of conductive material separated by a layer of flexible dielecrtic material.

6. An electric firing initiating device safeguarded against premature firing resulting from currents induced by high frequency energy comprising, in combination, a casing, an ignition assembly comprising a pair of spaced conductors, a heater element connected in bridging relation across said conductors and a heat sensitive ignition composition disposed in heat exchange relation to said heater element, a capacitor supported by said casing having the opposed electrodes thereof connected to said conductors respectively to electrically connect the capacitor in parallel relation to said heater element, said capacitor being formed of spaced tubular-shaped electrodes provided with an intermediate layer of dielectric material and said ignition assembly being disposed within the interior of said tubular capacitor, whereby alternating currents are by-passed through said device without affecting its sensitivity to direct firing currents.

7. An electric firing initiating device safeguarded against premature firing by currents induced therein resulting from high frequency energy comprising, in combination, a metallic casing, an ignition assembly disposed within said metallic casing comprising first and second spaced conductors, a heater element connected in bridging relation across said first and second conductors and a heat sensitive ignition composition disposed in heat exchange relation to said heater element, a. first capacitor supported by said casing having the opposed electrodes thereof electrically connected between said conductors respectively to electrically connect the first capacitor in parallel relation to said heater element, a second capacitor electrically connected between said metallic casing and said first conductor, and a third capacitor connected between said metallic casing and said second conductor.

8. An electric firing initiating device safeguarded against premature firing resulting from currents induced by high frequency energy comprising, in combination, a tubular casing, an ignition assembly comprising a pair of spaced conductors, a heater element connected in bridging relation across said conductors and a heat sensitive ignition composition disposed in heat exchange relation to said heater element, a tubular shaped capacitor supported within said casing having the opposed electrodes thereof connected to said conductors respectively to electrically connect the capacitor in parallel relation to said heater element, the axes of said tubular capacitor and said tubular casing being substantially coincident, whereby alternating purrents are by-passed through said capacitor to 11 safeguard the device without affecting its sensitivity to direct firing currents.

9. An electric firing initiating device safeguarded against premature firing resulting from currents induced by high frequency energy comprising, in combination, a tubular metal casing, an ignition assembly disposed within said casing comprising a pair of spaced conductors, a heater element connected in bridging relation across said conductors and a heat sensitive ignition composition disposed in heat exchange relation to said heater element, a tubular capacitor supported within said tubular metal casi'ng having the opposed electrodes thereof connected to said conductors respectively to electrically connect the capacitor in parallel relation to said heater element, the axes of said tubular capacitor and said tubular casing being substantially coincident, and said capacitor being insulated from said tubular metal casing by a sleeve formed of insulating material, whereby alternating currents are by-passed through said capacitor to safeguard the device without affecting its sensitivity to direct firing currents.

10. An electric firing initiating device safeguarded against premature firing resulting from currents induced by high frequency energy comprising, in combination, a metal casing having a tubular Wall, an ignition assembly disposed within said casing comprising a pair of spaced conductors, a heater element connected in bridging relation across said conductors and a heat sensitive ignition composition disposed in heat exchange relation to said heater element, a tubular shaped capacitor having spaced outer and inner opposed electrodes connected to said conductors respectively to electrically connect the capacitor in parallel relation to said heater element, said tubularwall of the metal casing forming the outer electrode of said capacitor, whereby alternating currents are by-passed through said capacitor to safeguard the device without affecting its sensitivity to direct firing currents.

11. An electric firing initiating device safeguarded against premature firing resulting from currents induced by high frequency energy comprising, in combination, a casing formed of an electrical insulating material having a tubular wall, an ignition assembly disposed within said casing comprising a pair of spaced conductors, a heater element connected in bridging relation across said conductors and a heat sensitive ignition composition disposed in heat exchange relation to said heater element, a tubular shaped capacitor having opposed electrodes separated by an insulating layer, said electrodes connected to said conductors respectively to electrically connect the capacitor in parallel relation to said heater element, said tubular wall of the insulating casing forming the insulating layer between the opposed electrodes of said capacitor, whereby alternating currents are by-passed through said capacitor to safeguard the device without affecting its sensitivity to direct firing currents.

References Cited in the file of this patent UNITED STATES PATENTS 2,086,527 Aughey et al. July 13, 1937 as: aw 

